The present invention relates to a discharge lamp with high lamp lumen maintenance. It finds particular application in connection with a ceramic metal halide (CMH) lamp with a source of available oxygen in the vessel that, during lamp operation, maintains a difference in solubility for tungsten species between the wall and the electrodes, and will be described with particular reference thereto.
High Intensity Discharge (HID) lamps are high-efficiency lamps that can generate large amounts of light from a relatively small source. These lamps are widely used in many applications, including highway and road lighting, lighting of large venues such as sports stadiums, floodlighting of buildings, shops, industrial buildings, and projectors, to name but a few. The term “HID lamp” is used to denote different kinds of lamps. These include mercury vapor lamps, metal halide lamps, and sodium lamps. Metal halide lamps, in particular, are widely used in areas that require a high level of brightness at relatively low cost. HID lamps differ from other lamps because their functioning environment requires operation at high temperature and high pressure over a prolonged period of time. Also, due to their usage and cost, it is desirable that these HID lamps have relatively long useful lives and produce a consistent level of brightness and color of light. Although in principle, HID lamps can operate with either an alternating current (AC) supply or a direct-current (DC) supply, in practice, the lamps are usually driven via an AC supply.
Discharge lamps produce light by ionizing a vapor fill material, such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel that maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficacies.
Such lamps often have a light output that diminishes over time due to blackening of the discharge vessel walls. The blackening is due to tungsten transported from the electrode to the wall. It has been proposed to incorporate a calcium oxide or tungsten oxide oxygen dispenser in the discharge vessel, as disclosed, for example in WO 99/53522 and WO 99/53523 to Koninklijke Philips Electronics N.V. Lamps produced according to the proposals in these applications may not, however, simultaneously meet acceptable lamp efficiency, color point, color stability, lumen maintenance, and reliability values for a commercial lamp.
The exemplary embodiment provides a new and improved metal halide lamp with improved lumen maintenance.